3. The Millward & Rivers requirement model: Qualitative aspects

One way of representing the model qualitatively is shown in
Figure 3. In its simplest form, it includes needs for new tissue and
maintenance. We view maintenance as a variable component, comprising both fixed
functional demands and variable regulatory extrinsic components. It is defined
operationally as the dietary intake needed to balance all losses, including
those which occur on feeding protein.

There are three components which are functional demands or
intrinsic. The first one is the fixed component relating to the net requirement,
i.e., the need to deposit new protein. Conceptually this is straightforward. The
other two are part of the maintenance needs. These are the intrinsic
requirements for obligatory amino acid consumption in non-protein pathways,
conceptually straightforward even if poorly understood, and the anabolic
drive.